While a group of deep sea divers are testing out new state-of-the-art equipment, an ocean lab, there's an underwater earthquake, a crevasse opens and the entire ocean lab falls in. Howeer, Dr. Samuel Andrews (Walter Pidgeon of VOYAGE TO THE BOTTOM OF THE SEA movie) and his chief diver MacKay (Ernest Borgnine) feel that they may be alive. So they take an experimental sub, lead by Cmdr. Adrian Blake (Ben Gazzara) down into the trench. Yvette Mimieux (THE TIME MACHINE) is also on board for eye candy. They are surprised when they go down into the trench and find giant plants and animals which look exactly like species they are familiar with.

The first part of this movie is great. There's real ships, real submersibles and people diving. Yet when they get into the experimental craft and are steering amongst the giant creatures, the miniature sub looks like it's being pulled along in someone's salt water aquarium. The only threatening scene is when MacKay goes outside and is pursued through seaweed by a huge striped tiger looking fish. Otherwise, I had a hard time suspending my belief in all of this.